In some instances, aircraft are authorized to execute instrument approaches and to land while being relatively close together. In this regard, some busy airport terminals permit modes of aircraft operation in which instrument approaches are executed for aircraft that are concurrently landing on substantially parallel runways, with the aircraft being permitted to be at the same altitude and to have less than one mile lateral separation therebetween. These instrument approaches and landings may be conducted under various conditions including in poor visibility conditions and with low cloud ceilings, such as in instrument meteorological conditions (IMC).
In instances in which aircraft that are landing on substantially parallel runways are executing simultaneous instrument approaches, separation is maintained between the aircraft in a procedural manner. In this regard, prior to the loss of standard radar separation between the aircraft, the aircraft are established on guidance to instrument approach procedures that have been designed so that the aircraft will not interfere with one another and that are deemed suitable for simultaneous use. In this regard, once the aircraft are established along their instrument approaches, the separation between the aircraft that is otherwise maintained, such as a vertical separation of one thousand feet and a lateral separation of three nautical miles, is discontinued. However, if either aircraft that is contemporaneously executing the approaches to parallel or near-parallel runways deviates from the lateral path defined by its assigned instrument approach, separation may no longer be assured procedurally and intervention by an air traffic controller may be indicated.
The International Civil Aviation Organization (ICAO) defines the standards for air traffic control authorities worldwide. ICAO requires air traffic controllers to identify such deviations of an aircraft from the lateral path defined by its assigned instrument approach during simultaneous independent operation in a timely manner to insure the continued safety of the air traffic. In this regard, air traffic controllers have the responsibility to recognize the penetration by an aircraft of a neutral zone that is considered to exist between substantially parallel runways. This neutral zone is defined as a non-transgression zone (NTZ) and the recognition by an air traffic controller of its penetration may permit the air traffic controller to take action to minimize the safety impact of any deviation by the aircraft. The NTZ is nominally defined as a region that extends laterally one thousand feet on either side of the median between parallel runways, although the NTZ may be defined differently based on the airport geometry.
In order to alert an air traffic controller of the penetration of the NTZ by an aircraft, a final monitor aid (FMA) system causes the NTZ to be depicted on the display of the air traffic controller radar system. The FMA also utilizes historical position data of an aircraft established by the prior locations of the aircraft along its flight path to identify an instance in which the aircraft is predicted to penetrate the NTZ and to correspondingly alert the air traffic controller. In domestic systems, for example, the FMA has three different alerting triggers. The FMA provides a caution alert in an instance in which the system predicts that an aircraft will enter the NTZ within ten seconds. In response to a caution alert, the radar target symbol and data block may change colors, such as from green to yellow, and an audible alert may be sounded. The FMA may also issue a warning alert in an instance in which the aircraft has penetrated the NTZ. In this instance, the target symbol and the data block may change to yet a different color, such as red. The FMA may also issue a surveillance alert in an instance in which a monitored aircraft has been in a coast state for more than three consecutive updates. In this instance, the target symbol and data block may also be caused to change colors, such as to amber.
The FMA alerting system assists air traffic controllers by providing notification of the penetration of the NTZ based on radar or other surveilled position data. Once notified, an air traffic controller may develop and issue instructions to the aircraft and the pilots of the aircraft should respond to those instructions so as to maintain the desired separation between the aircraft. The instructions issued by an air traffic controller may include instructions to an aircraft that is potentially endangered by another aircraft that has deviated into the NTZ so as to cause the other aircraft to alter its flight path to ensure its safety as well as instructions to the aircraft that has deviated in the NTZ to cause the deviating aircraft to return to its proper approach corridor.